The present invention relates to a plant for continuously regenerating sand which has previously been used in foundries for the manufacture of cores.
According to techniques generally used for the production of complicated metal components by melting, the molten metal is injected into modules containing cores which reproduce the internal shape of the components.
To go into more detail, special resins, for example of the phenolic type, and suitable catalysts which permit the curing thereof, are mixed with the sand in order to give the cores firmness.
In recent years, it has been found to be economically advantageous to recycle the sand used for the production of such cores, after removing the residues of resins, catalysts and other impurities contained therein.
Thus, plants have already been produced for the regeneration of foundry sand, as illustrated, for example, in EP-A-0 055 230. Some of those plants operate continuously and may comprise a combustion chamber, in which a combustible gas is added to the sand in order to enable the above-mentioned impurities to be burnt, and a cooling chamber, in which the sand coming from the combustion chamber is cooled to ambient temperature or slightly above ambient temperature.
According to a first known technique, those chambers, in which the sand is maintained in a fluidised state, are arranged inside separate containers between which the transfer of sand takes place by gravity, via suitable valves and without the provision of any additional device for thermal recovery.
That arrangement has the disadvantage of high operating, costs owing to the substantial consumption of energy (of the order of 50 m3 of combustible gas per tonne of treated sand). In addition, the fact that the cooling is carried out in a single stage means that it is of long duration, in order to enable the sand to be cooled from a temperature of approximately 700-800xc2x0 C., which is characteristic of the combustion stage, to the final desired temperature of approximately 30-35xc2x0 C.
Furthermore, the movement by gravity from one chamber to the other means that the plant must have a substantial vertical bulk (of the order of 7-10 m) which is not always compatible with the space available.
According to a further known technique, illustrated by EP-A-0 089 927, regeneration plants of the type indicated above are completed by thermal recovery devices which use the considerable heat of the streams of material leaving the combustion chamber to preheat the incoming streams of material.
Plants of the last-mentioned type are thus much more efficient in terms of energy than are the previous plants because they provide for a consumption of combustible gas of the order of 30 m3 per tonne of treated sand, but they are also much more expensive and functionally complex. This is due to the fact that the heat exchange devices, in addition to constituting additional components that have to be manufactured using valuable materials, such as stainless steels, are also intrinsically delicate, being subject to a very high degree of wear and thus requiring careful maintenance which involves not inconsiderable machine stoppage times.
Moreover, plants of the last-mentioned type also have the disadvantages of having a substantial bulk in the vertical direction and poor versatility.
In order to overcome the above-mentioned disadvantages of the known techniques, the present invention relates to a plant for regenerating foundry sand and to the associated method of operation which have the characteristics claimed specifically in the following claims.